The use of gait analysis in clinical settings has become wide spread. Evaluation of a patient's walking pattern requires the attachment of optical targets to the skin's surface. Data derived from monitoring the location of these targets while the patient walks provide the basis for objective human movement studies. In evaluating these data, the assumption that target positions indicate skeletal position is routinely ignored. The purpose of this study is to evaluate the magnitude of error produced by tracking the skeletal system using skin mounted targets. To date, the gait of three subjects has been tested using surface and bone mounted targets affixed to the shank. The results indicate that kinematic errors occur primarily about and along the long axis of the lower leg. It does not appear feasible to model these errors taking into account only the passive motion of soft tissue. Quantification of the errors associated with standard gait analysis procedures is an important component of the test validation process. The results of this study will have a significant impact on the future of clinical gait analysis.